1. Field of the Invention
The present invention relates to a water treatment apparatus to recover and recycle phosphorus contained in water to be treated. Particularly, the present invention relates to a water treatment apparatus to treat water generated in the process of treating excess sludge derived from biological wastewater treatment, such as activated sludge treatment, for treating sewage or industrial wastewater from, for example, food-processing plants.
2. Description of the Related Art
Now, it is predicted that phosphorus resources will be depleted worldwide in the future. However, Japan depends on imports for phosphorus resources, and therefore attention is being paid to a technique for recovering phosphorus contained in wastewater.
Phosphorus contained in wastewater becomes one of causes of eutrophication when discharged into the environment, and therefore it is absolutely necessary to remove phosphorus from wastewater and monitor the discharge of phosphorus into the environment. For this reason, until now, a technique for removing phosphorus from wastewater has been most actively studied. Representative examples of the technique for removing phosphorus include a biological phosphorus removal method using the ability of microorganisms to accumulate phosphorus and a coagulating sedimentation method.
In order to directly use such a phosphorus removal method as means for recovering and recycling phosphorus, it is necessary to additionally provide some treatment processes such as a process of incinerating excess sludge generated by biological phosphorus removal and a chemical treatment process using large amounts of chemicals. For example, in order to recycle recovered phosphorus as phosphorus resources, it is necessary to eliminate the influence of chemicals used and impurities derived from, for example, sludge, and therefore it is absolutely necessary to provide a process of preparing phosphorus resources or a process of purifying recovered phosphorus.
In view of such a technical background, recently, attention is being paid to a water treatment technique using a phosphorus adsorbent that selectively adsorbs phosphate anions. It is to be noted that in this specification, adsorbing phosphate anions is simply referred to as “adsorb phosphorus”, desorbing phosphate anions is simply referred to as “desorb phosphorus”, and recovering phosphate anions is simply referred to as “recover phosphorus”. Examples of an anion exchanger that selectively removes an anion such as phosphate anion include anion exchange resins and hydrotalcite-like inorganic layered compounds. Various methods for producing such an anion exchanger and various methods for recovering phosphorus using an advanced adsorbent utilizing the material characteristics of an anion exchanger have been developed.
For example, Jpn. Pat. Appln. KOKAI Publication Nos. 2007-260561 (Document 1) and 2008-49241 (Document 2) each disclose a phosphorus recoverion system using an adsorbent having the function of a hydrotalcite-like substance.
More specifically, Document 1 discloses a technique in which phosphorus contained in wastewater is adsorbed to a phosphorus adsorbent obtained by imparting the function of a hydrotalcite-like substance to an organic material, and then the phosphorus is desorbed from the adsorbent using a chemical solution, and then crystals of a phosphorus-containing salt are recovered from the chemical solution. Document 2 discloses a technique similar to the technique disclosed in Document 1, in which phosphorus contained in wastewater is adsorbed to an adsorbent containing a hydrotalcite-like material, and then the phosphorus is desorbed from the adsorbent to reuse the adsorbent.
However, in the cases of such conventional techniques disclosed in Documents 1 and 2, in addition to the need to impart phosphorus adsorption function to an organic material, a treatment system needs to have a structure to fix the phosphorus adsorbent, treatment cost increases due to an increase in, for example, production cost of the adsorbent, and it is absolutely necessary to treat the adsorbent to reuse it. Therefore, it is necessary to provide, after a phosphorus adsorption process, a process of desorbing phosphorus from the adsorbent using a large amount of a chemical. Further, as described above, since the adsorbent is obtained by imparting the function of adsorbing phosphorus to an organic material, it is difficult to increase the phosphorus content in the recovered adsorbent. In addition, it is also difficult to recycle the adsorbent having phosphorus adsorbed thereto as it is as phosphorus resources because the adsorbent contains the organic material.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2004-261729 (Document 3) discloses a multistage semibatch adsorption system using an adsorbent.
However, as in the cases of the conventional techniques disclosed in Documents 1 and 2, also in the case of the conventional technique disclosed in Document 3, the adsorbent is not recycled as resources but is disposed of or reused as an adsorbent, and further there is a problem in that if water to be treated contains suspended solids, the surface of the adsorbent is soiled with the suspended solids.